It is very desirable to produce thermal labels and thermal transfer labels that are linerless, so as to avoid the costs and difficulty associated with disposing of the release liner. Such labels are disclosed in U.S. Pat. No. 4,851,383. The labels described in that patent have a thermosensitive layer with dye for forming color images, and a silicone release coating. It is necessary to provide a barrier layer between the silicone release layer and the thermosensitive material, which increases the cost of production of the linerless labels. Also, the labels provided thereby are of questionable use for the food industry.
According to the present invention, linerless thermosensitive label stock is produced that may be readily utilized in the food industry, and which has a simplified construction compared to the prior art exemplified by U.S. Pat. No. 4,851,383. The term "thermosensitive" as used in the present specification and claims is intended to encompass both thermal transfer and thermal mechanisms.
According to one aspect of the present invention, linerless thermosensitive labels are produced by applying a release coating, of a material that has a low adherence to pressure sensitive adhesive that is part of the label stock, directly onto the thermosensitive layer, rather than requiring a barrier layer. The release coating is preferably a USDA approved coating. One particularly suitable coating which not only is USDA approved, but will not in any way adversely affect the thermosensitive label (as by tinting it a different color) contains as the primary operative ingredient chromium pentahydroxy (tetradecanoato) di-. This is available commercially under the trade name "QUILON C". The coating is preferably prepared by application then drying of a formulation containing between about 5-25 parts QUILON C, and about 75-95 parts water or alcohol.
Alternatively, the release coating may comprise a UV curable silicone, such as that available commercially from General Electric under the designation "G. E. 9300" and containing about 1-3% by weight photoinitiator (e.g. UV 9365C-D1).
The invention also contemplates a method of making a supply of linerless thermosensitive labels, from a substrate having a thermosensitive layer on a first face thereof, and a second face. The method comprises the following steps: (a) Continuously moving the substrate in a first direction. (b) Applying a pressure sensitive adhesive coating so that it covers the second face of the moving substrate. (c) Applying a release coating directly to the thermosensitive layer, the release coating having non-stick or low adhesion characteristics with respect to the pressure sensitive adhesive. (d) Drying the pressure sensitive adhesive and release coatings under conditions which do not cause significant adverse reaction of the thermosensitive layer (if necessary,if a hot melt adhesive was applied no drying equipment would be necessary). And, rolling the substrate into a roll, with the release coated second face on the outside of the roll. Step (d) may be accomplished by air drying at less than 150.degree. F., or by UV, radio frequency, or like non-destructive drying. Between steps (d) and (e), if the substrate is a web it may be slit into individual rolls, and perfed or die cut to form individual linerless labels in the roll. The rolls are tested for compliance with USDA requirements prior to use with food products, and printed, e.g. using a Hobart Scale.
According to another aspect of the present invention, a method of making a supply of linerless thermosensitive labels is provided which comprises the following steps: (a) Continuously moving the substrate in a first direction. (b) Applying a pressure sensitive adhesive coating so that it covers the second face of the moving substrate. (c) Applying a release coating so that it covers the thermosensitive layer, by making a formulation comprising or consisting essentially of about 5-25 parts chromium pentahydroxy (tetradecanoato) di- and about 75-95 parts water or alcohol, and applying that formulation so that it covers the thermosensitive layer. (d) Drying the pressure sensitive adhesive and release coatings under conditions which do not cause significant adverse reaction of the thermosensitive layer. (Steps (b) and (c) can be performed in any order, with the drying steps simultaneously, or at different times). And, then (e) rolling the substrate into a roll, with the release coated second face on the outside of the roll.
It is the primary object of the present invention to simply yet effectively produce linerless thermosensitive label stock, including stock that can be used on food products. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.